1. Field of the Invention
The present invention relates to two-port isolators, and in particular, to a two-port isolator for use in microwave bands, a characteristic adjusting method therefor, and a communication apparatus including a two-port isolator.
2. Description of the Related Art
In general, two-port isolators allow signals to pass through them only in a transmitting direction and prevent the signals from passing through them in a reverse direction. The two-port isolators are used in transmitting circuit portions of mobile communication apparatuses, such as automobile telephones and cellular phones.
The isolator disclosed in Japanese Unexamined Patent Application Publication No. 2003-46307 is a known two-port isolator of the type described above, that is, a type of isolator including first and second center electrodes. FIG. 20 shows a two-port isolator 300 as disclosed in the above publication. The two-port isolator 300 includes a ferrite member 303, two center electrodes 301 and 302 which are disposed on an upper surface of the ferrite member 303 and which have an intersection angle Φ between 40 and 80 degrees, matching capacitors C11 and C12, a parallel capacitor Cw, and a resistor R. An inductor L1 defined by the first center electrode 301, and the matching capacitor C11 define a parallel resonant circuit. An inductor L2 defined by the second center electrode 302, and the matching capacitor C12 define a parallel resonant circuit. The two-port isolator 300 also includes an input terminal 311, an output terminal 312, and a ground terminal 313.
The two-port isolator 300 has an advantage in that a high attenuation is obtained even outside an operating frequency range because the first and second center electrodes 301 and 302 are perpendicular to each other. In the two-port isolator 300, one end of the first center electrode 301 is used as an input port P1, one end of the second center electrode 302 is used as an output port P2, and the other ends (common end) of the first and second center electrodes 301 and 302 are used as a ground port P3. The two-port isolator 300 has a problem in that, when a signal is conveyed from the input terminal 311 to the output terminal 312, the two resonant circuits resonate to produce a large insertion loss.
Accordingly, to eliminate this problem, a low-loss two-port isolator is disclosed in Japanese Unexamined Patent Application Publication No. 9-232818. As FIG. 21 shows, a two-port isolator 320 of this type includes a ferrite member 323, two center electrodes 321 and 322 which are disposed on an upper surface of the ferrite member 323 and which have an intersection angle θ of about 90 degrees, matching capacitors C11 and C12, and a resistor R. An inductor L1 defined by the center electrode 321, and the matching capacitor C11 define a parallel resonant circuit. An inductor L2 defined by the center electrode 322, and the matching capacitor C12 define a parallel resonant circuit. The two-port isolator 320 also includes an input terminal 331, an output terminal 332, and a ground terminal 333.
In the two-port isolator 320, one end of the first center electrode 321 is used as an input port P1, one end of the second center electrode 322 is used as a ground port P3, and the other ends of the first and second center electrodes 321 and 322 are used as an output port P2. In the two-port isolator 320, when a signal is conveyed from the input terminal 331 to the output terminal 332, a resonant circuit (defined by the inductor L1 and the matching capacitor C11) between the input port P1 and the output port P2 does not resonate. Only one resonant circuit (defined by an inductor L2 and matching capacitor C12 connected between the output port P2 and the ground port P3) resonates. Thus, in the two-port isolator 320, an insertion loss is reduced.
In general, an input admittance Y12 of a two-port isolator is normally designed to be 0.02 S+0 j S, and its susceptance part is 0 S. In terms of impedance, an input impedance Z12 of the two-port isolator is normally designed to be 50Ω+0 jΩ. However, in the case of mounting the two-port isolator on an actual circuit board of a mobile communication apparatus, the two-port isolator is affected by a pad capacitor on a surface on which the two-port isolator is mounted, lines connected to other components, circuit elements, etc. Accordingly, in relation to an input terminal of the two-port isolator, the susceptance part of the admittance Y11 is not always 0 S. In many cases, it has a positive value (capacitive) or a negative value (inductive).
In addition, in order to enable maximum power to pass in the two-port isolator by reducing a power loss at an input terminal of the isolator, the input admittance Y12 must be matched so as to be a complex conjugate of the admittance Y11. In other words, the susceptance part of the admittance Y12 must be inductive or capacitive in accordance with the susceptance part of the admittance Y11.
In the two-port isolator 300 shown in FIG. 20, the matching capacitor C11 is connected in parallel with the center electrode 301 between the input terminal 311 and the ground. Therefore, by adjusting the capacitance of the matching capacitor C11, the admittance Y12 is easily matched so as to be a complex conjugate of the admittance Y11.
Conversely, in the two-port isolator 320 shown in FIG. 21, no matching capacitor is connected in parallel with the center electrode 321 between the input terminal 331 and the ground. Accordingly, adjustment as in the above two-port isolator 300 is impossible. A matching capacitor could be connected to the input terminal in parallel with the center electrode 321. However, an increase in the number of circuit elements prevents reduction in size and cost. In addition, an increase in the number of connecting points between circuit elements reduces reliability.